272 E. W. MACBRIDE. 



larvse of Stage D, aud saw the completely closed coelomic 

 vesicle on the right, and the imperfect transverse septum on the 

 left side, and was at a loss how to interpret these appearances ; 

 the right hydrocoele he calls a mesenchymatous vesicle. 



It is curious to see how unable many zoologists have been to 

 grasp Bury's idea of the anterior coelom ; thus Seeliger, who 

 has confirmed his work on Antedon and amplified it till it 

 may be said that we have an exhaustive knowledge of the 

 subject, objects to consider the structure Bury named anterior 

 coelom as such, on the supposition that Bury meant by that a 

 fellow of the hydrocoele, which it obviously is not. Seeliger 

 calls it the " parietal canal," but the structural facts he so 

 accurately relates are convincingly in favour of Bury's inter- 

 pretation. The weak point in Bury's observations on Plutei 

 and other larvse was that in no case were any more than a few 

 stages taken at random examined ; but I hope the account I 

 have given in this paper will provide a more solid basis for the 

 idea of segmentation of the coelom in Echinoderms. Field (5) 

 has published a short paper on the development of the Bipin- 

 naria; he carries it up only to a stage corresponding to midway 

 between Stages B and C of Asterina. The chief points of 

 interest in the paper are that many of the larvae had two 

 madreporic pores, and he suggests that this is a normal stage 

 in the ontogeny ; also that the two ciliated rings characteristic 

 of the Bipinnaria are derived from one, and that there is a 

 prseoral sense-organ comparable to that in Antedon. 



This paper does not contain the discovery that the water- 

 vascular rudiment is paired ; for, as a matter of fact, in the 

 oldest larva examined no trace of the left hydrocoele was 

 present. The " schizocoelic space," near the madreporic pore, 

 may represent the rudiment of the right hydrocoele ; needless 

 to say, it was not recognised as such. 



Theel (22) has recently succeeded in following the meta- 

 morphosis in Echinocyamus pusillus so far as the external 

 features are concerned. He finds that already in the blastula 



M. Uusso's technique was obviously not equal to dealing successfully with 

 such difficult subjects as Echinoderm larvae. 



